Magneto-inductive flow measuring devices utilize, for volumetric flow measurement, the principle of electrodynamic induction, wherein charge carriers of the medium moved perpendicularly to a magnetic field induce a measurement voltage in measuring electrodes likewise arranged essentially perpendicularly to the flow direction of the medium. The measurement voltage induced in the measuring electrodes is proportional to the flow velocity of the medium averaged over the cross section of the measuring tube; it is thus proportional to volume flow. If the density of the medium is known, then the mass flow in the pipeline, or in the measuring tube, can also be ascertained. The measurement voltage is usually tapped via a measuring electrode pair arranged in the region of maximum magnetic field strength, where, thus, maximum measurement voltage is to be expected. The measuring electrodes are coupled with the medium either galvanically or capacitively.
Usually, magneto-inductive flow measuring devices are mounted via flanges into the pipeline, in which the volume flow rate of the medium is to be ascertained. The flange connections are welded pipe-flange connections, loose-flange connections, or wafers, which are mounted between two flanges secured to the pipeline. While, thus, in the case of the first two embodiments, flanges are affixed in the end regions of the measuring tube of the magneto-inductive flow measuring device, the flanges in the case of the third embodiment are mounted on the pipeline; via clamping screw-connections, the wafer is mounted between the flanges of the pipeline.
In the case of welded pipe, or tube, to flange connections, the flanges are pushed onto the two pipe, or tube, ends and welded in the desired position. Then, the pipe, or tube, is washed, ground smooth in the flange areas and galvanized for corrosion protection. There follows then the providing of the measuring tube with a liner. In order that a high-quality connection be achieved between the flanges and the measuring tube, the inner diameter of a flange must fit the outer diameter of the measuring tube within close tolerance limits. Since, in the case of this known solution, relatively close tolerance limits must be maintained for the flanges and the pipes, material costs are relatively high. Beyond that, flanges and measuring tube must be made of weldable materials.
The manufacture of wafers is similar. Wafers are measuring devices, which do not have flanges. In the case of the known manufacture of wafers, angle end-pieces are welded on the measuring tube. The angling is achieved, for example, via a deep draw method or via a flaring method. In turn, the welded joints must be ground smooth, cleaned and galvanized.
In the case of loose-flange connections, the flanges are not fixedly connected with the measuring tube, but, instead, pushed loosely onto the measuring tube. For axial affixing of the loose flanges in the installed state of the flow measuring device in the pipeline, angled or flared end-pieces are welded onto the measuring tube. Due to the welding process, it is, also here, necessary to smooth the welded joints by grinding, to clean them, and apply an anti-corrosion coating. Therefore, a magneto-inductive flow measuring device with loose flanges, likewise, requires relatively long manufacturing times and is, thus, costly to manufacture.